Chapter 12

Newton’s first law of motion - an object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.

Objects tend to maintain their state of motion.

Inertia - the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.

Inertia is related to an object’s mass. Mass is a measure of inertia. Causes all objects to fall with the same acceleration regardless of mass

Seat belts and car seats provide protection.•Because of inertia, you slide toward the side of a car when the driver makes a sharp turn.

•When the car you are riding in comes to a stop, your seat belt and the friction between you and the seat stop your forward motion.

Forces and motion are connected

An object will have greater acceleration if a greater force is applied to it

The mass of an object and the force applied to it affect acceleration

Newton’s 2nd Law of Motion – states that the unbalanced force acting on an object equals the object’s mass times its acceleration. It connects force, mass, and acceleration in the equation a = f / m, acceleration = net force / mass, same as f = ma

Force is measured in newtons (N).1 N = 1 kg 1 m/s2

Newton’s Second Law Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is 0.657 m/s2. What is the unbalanced force necessary to produce this acceleration of the lion and the stretcher?

1. List the given and unknown values.

Given: mass, m = 175 kgacceleration, a = 0.657 m/s2

Unknown: force, F = ? N

2. Write the equation for Newton’s second law.force = mass accelerationF = ma

3. Insert the known values into the equation, and solve.F = 175 kg 0.657 m/s2

F = 115 kg m/s2 = 115 N

Newton’s 2nd law can also be stated as:The acceleration of an object is proportional to the net force on the object and inversely proportional to the object’s mass.

Friction – force that opposes motion between two surfaces that are touching each other◦ Microwelds – areas where surface bumpers stick

together, are the source of friction Types of Friction:

◦ Static friction – friction b/n two surfaces that are not moving past each other

◦ Sliding friction – force that opposes the motion of two surfaces sliding past each other

◦ Rolling friction – friction b/n a rolling object and the surface it roll on

◦ Fluid friction – force that opposes motion in a fluid

Air resistance that opposes the force of gravity

The amount of air resistance depends on an object’s shape, size, and speed

Terminal velocity – forces on a falling object are balanced and the object falls with constant speed◦air resistance is equal in magnitude and opposite in direction to the force of gravity

Velocity is constant when air resistance balances weight.

All matter is affected by gravity.•Two objects, whether large or small, always have a gravitational force between them.

•When something is very large, like Earth, the force is easy to detect.

Gravitational force increases as mass increases. Gravitational force decreases as distance

increases

Free fall is the motion of a body when only the force of gravity is acting on the body.

Free-fall acceleration near Earth’s surface is constant.•If we disregard air resistance, all objects near Earth accelerate at 9.8 m/s2.

•Freefall acceleration is often abbreviated as the letter g, so g = 9.8 m/s2.

Law of Gravitation – any two masses exert an attractive force on each other, F = G (m1m2)/d2

G = 6.67 x 10-11 m3/kg.s2

Acceleration due to gravity on Earth is 9.8 m/s2

Mercury – 3.8 m/s2

Jupiter – 25.8 m/s2

Gravity is one of the four basic forces that also include the electromagnetic force, the strong nuclear force, and the weak nuclear forceGravity is a long-range force that gives the universe its structure

Weight is equal to mass times free-fall acceleration.

weight = mass free-fall accelerationw = mg

Weight is different from mass.•Mass is a measure of the amount of matter in an object.

•Weight is the gravitational force an object experiences because of its mass.

Weight influences shape. • Gravitational force influences the shape of living

things.

Weight – gravitational force exerted on an object

Decreases as an object moves away from the Earth

Weight results from a force; mass is a measure of how much matter an object contains

Objects in the space shuttle float b/c they have no force supporting them

Orbiting objects are in free fall.

The moon stays in orbit around Earth because Earth’s gravitational force provides a pull on the moon.

Two motions combine to cause orbiting.

Projectile motion is the curved path an object follows when thrown, launched, or otherwise projected near the surface of Earth.

Projectile motion applies to objects that are moving in two dimensions under the influence of gravity.

Projectile motion has two components—horizontal and vertical. The two components are independent.

Projectiles have horizontal and vertical velocities due to gravity, and follow a curved path

Centripetal acceleration – acceleration toward the center of a curved path

Centripetal force – an unbalanced force, causes centripetal acceleration

Newton’s 3rd Law of Motion – to every action force there is an equal and opposite reaction force

Action-Reaction forces – act on different objects and differ from balanced forces

Rocket propulsion is based on Newton’s 3rd law of motion

Neptune was discovered based on predictions of gravitational forces and Newton’s laws

Momentum – related to how much force is needed to change an object’s motion; p= mv, momentum = mass x velocity

F = (mvf – mvi) / t - changing momentum formula

Law of Conservation of Momentum – momentum can be transferred between objects; momentum is not lost or gained in the transfer